1. Field of the Invention
The disclosed technology relates to a configuration, a sensing element with such configuration, an electrochemical sensor comprising such a sensing element and a method for electrochemical sensing using such an electrochemical sensor.
2. Description of the Related Technology
Electrochemical sensing relies on oxidation reactions or reduction reactions involving a target analyte (a gas or a liquid), at an electrode/electrolyte interface. The established electrochemical potentiometric and amperometric sensors are physically limited in sensitivity, response dynamic and selectivity due to the electrode created potentials and redox processes taking place at the electrode/electrolyte interface. Sensor miniaturization additionally causes a smaller signal, a reduced selectivity and sensitivity, an increase of the noise level, and reduces the long term stability.
In two-electrode potentiometric sensors the difference in electrochemical potential between a reference electrode and a working electrode is measured while no current is flowing in the electrolyte between the electrodes. In amperometric sensors (usually comprising three electrodes) a current is flowing between a working electrode and a counter electrode through the electrolyte.
Electrochemical sensors with good sensitivity have been reported based on ion sensitive field effect transistors (ISFETs), e.g. GaN/AlGaN ISFETs, having an open gate in direct contact with the electrolyte. For such ISFET based devices, sensing is based on monitoring a conductivity change of the source-drain channel. They operate in a pseudo-amperometric mode, wherein a current is flowing between a source and a drain of the transistor (not through the electrolyte). These GaN/AlGaN-based electrochemical sensors are relatively complex and the pseudo-amperometric mode leads to relatively high power consumption. The ISFET-mode excludes established electrochemical two- and three-electrode layouts thus preventing potentiometric low power implementations.
GaN/AlGaN 2DEG-based electrochemical ISFET-sensors are much more sensitive compared to non-2DEG-based electrochemical sensors. This is related to the very high sensitivity of the two dimensional electron gas (transistor channel) to any surface potential charge, which electrostatically interacts with the electrons in the channel and thereby modulates the 2DEG-channel resistance. Surface charge may be generated by e.g. redox processes involving a functional layer deposited at the surface, by diverse molecular getter-effects or by an electrochemical double-layer at the surface/electrolyte interface. These devices can be used for sensing of gases, polar molecules, (bio-)molecules or (bio-)reactions, pH, and the concentration of ions in solutions.
A drawback of such (2DEG) ISFET based configurations is that they require the presence of a source-drain current for sensing, such that they do not allow zero-current, and thus zero-power, potentiometric applications.